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Apparent Recovery and Physiological Efficiency of Nitrogen in Sesbania Incorporated before Rice

Multiple Cropping Dep., Int. Rice Res. Inst. (IRRI), P.O. Box 933, Manila, Philippines
Dep. of Soil Sci., 202 Strand Agric. Hall, Oregon State Univ., Corvallis, OR

^* Corresponding author.

When incorporated for lowland rice (Oryza saliva L.), the agronomic efficiency of green manure N exceeding 100 kg ha^–1 is low. Apparent N recovery and physiological efficiency of green manure N, ranging to 200 kg N ha^–1 were determined on a fine, mixed, isohyperthermic Aerie Tropaqualf in the Philippines. Sesbania sp. accumulated 100 kg N ha^–1 in 48 d and 160 kg N ha^–1 in 60 d. Nitrogen uptake (NU, kg N ha^–1) from green manures flooded during the last 25 d before incorporation was a linear function of incorporated N (NU = 49 + 0.23N). Uptake from green manures that were not flooded diminished as incorporated N increased (NU = 35 + 0.51N - 0.0012N²). The flooding effect was attributed to inhibition of NO^–₃ accumulation on submerged control plots. Nitrogen uptake was linearly related to extractable NH⁺₄-N determined 7 d after green manure incorporation. As N uptake increased, however, physiological efficiency declined. Dry-season rice recovered only 6% of the green manure N not recovered by wet-season rice. Because both apparent recovery and physiological efficiency are low, and residual green manure N recovery is minor, the advantage to growing Sesbania sp. for more than 48 d to accumulate N in excess of 100 kg ha^–1 is limited.

Received for publication February 16, 1988.